1. Field of the Invention
The present invention relates to a communication network which performs communication from a plurality of transceivers to a single receiver using synchronous code division multiplexing, and more particularly to a technique for establishing synchronism in this communication. The present invention may be applied to a PON (Passive Optical Network) type optical communication network, for example.
2. Description of Related Art
A network referred to as FTTx is known as an example of an optical communication network. FTTx is a communication network for connecting an accommodating apparatus to a subscriber building (a house, an office, or the like) using optical fiber. FTTH (Fiber To The Home) is known as an example of FTTx. FTTx is used as an access system for connecting a terminal computer of the subscriber to the Internet, for example.
A PON (Passive Optical Network) is known as an example of a technique for realizing FTTx. In a PON, a plurality of communication terminals (ONUs; Optical Network Units) are connected to a single optical communication path which is connected to an accommodating apparatus (OLT; Optical Line Terminal) via an optical coupler. A PON realizes low-cost broadband communication by sharing the single optical communication path among the plurality of ONUs.
On a shared communication path, an uplink direction (i.e. from the optical coupler to the OLT) communication channel and a downlink direction (i.e. from the OLT to the optical coupler) communication channel are divided by Wavelength Division Multiplexing (WDM). The downlink direction communication channel and uplink direction communication channel are respectively shared among the plurality of ONUs using a technique such as Time Division Multiplexing (TDM) or Code Division Multiplexing (CDM).
An STM (Synchronous Transfer Mode)-PON, an ATM (Asynchronous Transfer Mode)-PON, and an Ethernet (registered trademark)-PON are known as examples of PONs using TDM. In an STM-PON, the communication time of each ONU is allocated in a fixed manner. In an ATM-PON, the communication time allocated to each ONU is varied dynamically in accordance with the amount of communication data. An Ethernet-PON uses a LAN (Local Area Network) switching technique as a technique for sharing the communication path. An Ethernet-PON is disclosed in “Ethernet PON System”, H. Ohnishi et al, Fujikura Giho No. 102, pages 18 to 21, April 2002, for example.
Synchronous CDM and asynchronous CDM are known as types of CDM. A synchronous CDM-PON multiplexes synchronized communication data. On the other hand, an asynchronous CDM-PON multiplexes communication data that have not been synchronized. Synchronous CDM is disadvantaged in that processing is required to synchronize the communication data but advantaged in that the effects of interference are small. Therefore, a synchronous CDM-PON is capable of securing an extremely high transmission efficiency.
Downlink direction synchronous CDM is realized by simple control. The OLT synchronizes, multiplexes, and transmits communication frames storing header information for identifying a destination. Each ONU determines the destination using the header information in the received communication frames, and scraps communication frames addressed to other ONUs.
In contrast, uplink direction synchronous CDM requires complicated control. The reason for this is that the transmission phase of the communication frames must be adjusted among the ONUs. Typically, the optical distances of the optical communication paths connecting the respective ONUs to the optical coupler are different to each other. Accordingly, the transmission times between the respective ONUs and the optical coupler also differ from each other. Therefore, phase adjustment of the communication frames transmitted respectively by each ONU is extremely complicated.